The present invention relates to a solid activated carbon used as an adsorbent and a process for manufacturing the same. The present invention relates, particularly, to a solid activated carbon useful as an electrode for an electric double layer capacitor and to a process for manufacturing the solid activated carbon. The present invention also relates to an electric double layer capacitor using the solid activated carbon as an electrode.
A solid activated carbon is utilized as an adsorbent used for deodorizing, removal of harmful gas, and solvent recovery. The solid activated carbon is also useful for the refining of drinking water or foods and is adaptable to filter media used in wastewater treatment. It is necessary for the solid activated carbon to have a large specific surface area and high strength depending on its use, when it is used as adsorbents or filter media.
The solid activated carbon is particularly suitable to a pair of electrodes used for an electric double layer capacitor. The electric double layer capacitor is, as shown by the sectional structure in FIG. 1, provided with a pair of polarizable electrodes 1 composed of activated carbon with a specific form and with a liquid electrolyte 2 disposed between the pair of electrodes 1, making use of an electric double layer produced at the interface between the inner wall of a great number of pores in the activated carbon electrode 1 and the electrolyte 2 penetrating into the pores.
The activated carbon is porous with the total area of pores per unit volume, i.e. a specific surface area being large and hence the capacitance per unit volume of the capacitor is very large. It is also required for the electrode of the capacitor to possess, besides a large capacitance, low electric resistance of the electrode and to be highly strengthened with reduced occurrence of cracks or damages.
The electric double layer capacitor making use of the activated carbon as an electrode has a large capacitance. It can be also applied to batteries used as, for instance, an auxiliary power source for a large capacity motor, backup power source for a memory, or power source for vehicles.
There are many traditional processes for producing the solid activated carbon as follows:
(i) A process in which a kneaded mixture consisting of activated carbon, carbon black, micropowdery carbon, or electroconductive carbon and ethylene tetrafluoride (PTFE) resin, or other fluorine-containing polymer resins is formed into a sheet by means of roll forming, compression, extrusion, rolling, drawing, or combinations of these means to prepare solid activated carbon (see Japanese Patent Publication JP-A 62-200715, JP-A 63-17311, JP-A 63-173311, JP-A 5-121269, and JP-A 5-283287).
(ii) A process in which a carbon compound is carburized to prepare a carburized substance, which is subjected to a primary activation process to make a carbon base material and a binder is added to the carbon base material to form a molded compact, which is then carburized followed by secondary activation to obtain a plate activated carbon (see Japanese Patent Publication JP-A 8-83736).
(iii) A process in which a mixture of an activated carbon powder or activated carbon fiber, a granulated or powdery phenol resin, and a powdery or fibrous thermoplastic resin is shaped and heat-treated in a non-oxidizing atmosphere to obtain a solid activated carbon (see Japanese Patent Publication JP-A 7-99141).
(iv) A process in which a phenol resin molded compact is foamed, carburized, and activated to form a porous carbon compact, or in which mesophase pitch is then foamed, carburized, and activated to produce a porous molded compact (see Japanese Patent Publication JP-A 2-297915).
(v) A process in which only an activated carbon micropowder, or a mixture of the carbon micropowder, a carbon fiber or activated carbon powder and a mesophase carbon, is baked under pressure to produce a solid carbon (see Japanese Patent Publication JP-A 3-132009).
(vi) A process in which a mixture of an activated carbon powder and a powdery phenol resin is injection-molded, followed by heat treating to produce a solid activated carbon/carbon complex (see Japanese Patent publcation JP-A 6-45189).
(vii) A process in which a globular carbon material produced by carburizing a curable globular phenol resin and a thermally reactive globular phenol resin are mixed, a metallic mold is filled with the mixture to heat-cure the mixture under pressure, and the shaped product is heat-treated under an inert atmosphere and activated (see Japanese Patent pulication JP-A H6-69075, JP-A 6-69076, and JP-A 6-69077).
Such a solid activated carbon produced by the conventional production methods more increases in the specific surface area and thereby in the adsorption with an increase in an activated carbon component. The strength, on the contrary, decreases. In order to achieve a desired strength, the proportion of activated carbon in the compact is necessarily reduced.
In sum, the aforementioned solid activated carbon is produced by molding a mixture consisting of a porous activated carbon with a specific surface area increased to the extent that the durability and the mechanical strength are not impaired and various organic resin binders, and by carburizing the molded compact in a non-oxidizing atmosphere to unite the activated carbon and the resin binders. The amount of the activated carbon to be filled is limited to a fixed range in relation to the total of pores volume.
To improve the strength of the solid activated carbon, a large amount of the organic resin binder is required. This increases the potential that the pores of the activated carbon are blocked with the binder resin because of excellent wettability of the binder resin, which, coupled with the limited amount of the activated carbon, more reduces the specific surface area.
A resin binder having inferior forming characteristics, on the other hand, lacks potential to block the pores of the activated carbon because of its high viscosity and poor wettability. Such a resin binder, however, has the inferior endurance problem that the electrode is broken during the production process whereby the production yield is reduced or the electrode is broken during use whereby the function of the electric double layer capacitor is not fulfilled because of the reductions in the strength of the resulting molded compact and in the mechanical strength of the product after the carburizing heat treatment to unite the binder with the activated carbon.
The conventional solid activated carbon is also low in the proportion of activated carbon, posing the problems of low adsorption and reduced mechanical strength when it is utilized as an adsorbent used in deodorizing equipment, harmful gas-removing equipment, and for solvent recovery.
To be concrete, no conventional production processes allows production of solid activated carbons having an apparent density of 0.5 g/cm3 or more, a strength of 300 gf/mm2 or more, and a specific surface area of 500 m2/g or more.
The solid activated carbon cannot be also mass-produced in the conventional process, giving rise to the problem of high production costs.
When, for instance, such a solid activated carbon is used as an electrode to form an electric double layer capacitor, the capacitance is smaller than the practical range and a large internal resistance results.
There have been therefore a demand for a solid activated carbon increased in the specific surface area of the pores, to the extent that the durability is not impaired, to increase the capacitance per unit volume of an electrode.
In addition, the capacitance of the solid activated carbon to be used as an electrode which is measured according to a constant-current discharge method reduces when the current density is as low as 30 mA/cm2 as compared with the case of discharging at a current density as high as 300 mA/cm2. When the capacitor is applied to such a battery for a power source which consumes electric power at a low current, the capacitance is more insufficient.
An object of the present invention is to provide a solid activated carbon which maintains a large specific surface area and has high mechanical strength.
Another object of the present invention is to provide a porous carbon electrode material containing a high proportion of activated carbon and therefore having high mechanical strength.
Still another object of the present invention is to provide a process for manufacturing a solid activated carbon capable of maintaining a large specific surface area and having high mechanical strength.
A further object of the present invention is to provide a process for manufacturing a porous carbon electrode material containing with a high proportion of activated carbon and therefore having high mechanical strength.
A still further object of the present invention is to provide a capacitor using a solid activated carbon to increase the capacitance of the capacitor.
A still further object of the present invention is to provide an electric double layer capacitor using a solid activated carbon, particularly a capacitor which is increased in the capacitance when discharging at a low current.
According to a first aspect of the present invention, there is provided a solid activated carbon comprising an activated carbon powder and/or an activated carbon fiber and a carburized substance produced by heat-treating a polymer of vinyl alcohol or its derivative.
Since the carburized substance contains the above polymer carburized substance, particularly mesophase carbon of a polymer, the solid activated carbon is formed of the combined powder or fiber firmly combined by the carburized substance while maintaining the porosity of the powder and/or fiber of activated carbon, ensuring a desirable specific surface area and strength.
According to a further aspect of the present invention, there is provided a solid activated carbon comprising an activated carbon powder and/or an activated carbon fiber and 0.5-10% by weight of a silicon (Si). In the invention, an activated carbon powder and/or an activated carbon fiber, an organic binder and a Si-containing compound are compounded. In a step of heat treatment, Si binds the activated carbon powder and/or fiber to reinforce the solid activated carbon. With the addition of the Si-containing compound and a small organic binder, a solid activated carbon having high strength and a large specific surface area can be obtained. As the Si-containing compound, high-silicate glass or organic silane derivatives may be used.
As for the solid activated carbon formed in the present invention, the size distribution of the pores of the activated carbon is defined so as to increase the capacitance of an electric double layer capacitor particularly in a discharging operation at a low current. The pore size distribution is designed in such a manner that the ratio of the volume of the pores with a size of 15 angstroms or less is 65% or more for the total volume of pores and the pore size giving the maximum differential pore volume is 8 angstroms or less. In the present invention, the pore size distribution is allowed to be near the distribution of the order of the size of the electrolyte ion thereby delaying discharge at a low current density to increase the capacitance.